The purpose of these studies is to establish a better understanding of the energy metabolism in tissues, in vivo. Towards this goal, the laboratory concentrates on the use of non- invasive and non-destructive techniques to evaluate the biochemical and physiological function of the heart and skeletal muscle with regard to energy metabolism. The following major findings were made over the last year: 1) Using an optical window in the 540 to 580 nm region in the in vivo porcine heart, oxygen delivery to the heart was evaluated using the myoglobin and cytochrome absorption spectrum. These studies reveal that myoglobin does not significantly contribute to cellular oxygen transport in the intact heart as previously believed. Theoretical models confirm that tissue oxygen tensions in the heart remain well above the P50 for myoglobin except under extreme oxygen demands or ischemia reducing the possiblity of oxygen playing a significant role in the regulation of oxidative phosphorylation or blood flow within the heart. 2)Using new optical and ion selective electrodes techniques the effect of calcium ions (Ca) on the ATP sythesis pathways in isolated porcine heart mitochondria was established. It was found that Ca activates not only the dehydrogenases associated with NADH generation but also the F1-Fo-ATPase directly in intact mitochondria. More than 60% of the effect of Ca on ATP production was found to be due to the direct effects of Ca on the F1-Fo-ATPase. This regulatory action of cytosolic Ca on ATP production may be an important element in the balance of cardiac energy metabolism with workload in vivo. 3) Using confocal fluorescence microscopy the distribution of mitochondrial NADH within single cardiac myocytes has been determined. Monitoring this key metabolic intermediate in intact myocytes will hopefully lead to a better understanding of cellular compartmentation in metabolic regulation of the heart. 4) Using 31P NMR and a specially constructed exercise device, the effects of muscle action, eccentric or concentric, was evaluated on the energy metabolism of human skeletal muscle. It was found that the ATP production capacity of muscle increases with increases in metabolic strain, independent of the type of muscle action. Simple feedback models of ATP hydrolysis products on oxidative phosphorylation were found to be inadequate in describing the metabolic activation associated with exercise in these studies.